Micromechanical component and corresponsing production method

ABSTRACT

A micromechanical component including a chip which is mounted on a substrate and has an encapsulated chip area which is higher than its vicinity, as well as a mounting area provided in the vicinity of the encapsulated chip area. The chip being mounted on the substrate by a mounting arrangement which is connected to the mounting area, so that the encapsulated chip area faces the substrate and is positioned at a distance therefrom. The encapsulated chip area is surrounded by an underfill beneath the chip. A method for the manufacture of the micromechanical component is also provided.

FIELD OF THE INVENTION

The present invention relates to a micromechanical component whichincludes a substrate-mounted chip having an encapsulated chip area whichis higher than its vicinity and a mounting area provided in the regionof the encapsulated chip area, as well as a method for manufacturing themicromechanical component.

BACKGROUND INFORMATION

The structure of a functional layer system and a method for the hermeticencapsulation of sensors by a surface micromechanical arrangement isdiscussed in German patent document no. 195 37 814. This publicationdescribes the manufacture of the sensor structure using availabletechnological methods. The above-mentioned hermetic encapsulation isachieved via a separate cap wafer made of silicon, which is structuredaccording to complex structuring processes, for example KOH etching. Thecap wafer is applied to the substrate having the sensor (sensor wafer)by glass soldering (seal glass). For this purpose, a wide bonding framemust be provided around each sensor chip to ensure adequate adhesion andsealing of the cap. This greatly limits the number of sensor chips persensor wafer. The great space requirements and complex cap wafermanufacturing process make the sensor encapsulation very expensive.

An alternative encapsulation technique is discussed in European patentdocument no. 0 721 587, which refers to a layer structure in which thestructured trenches of a micromechanical component, for example acapacitive acceleration sensor, are covered by or filled with aninsulating material. A membrane layer is applied to this insulationlayer and structured so that window openings are provided over themoving elements of the component structure. The insulating material anda lower sacrificial layer located beneath the functional layer of thecomponent structure are selectively etched through these window openingsagainst the perforated membrane layer and the functional layer. Thewindow openings in the membrane layer are then covered by a cover layer,thereby forming a hermetically sealed cavity above the moving elements.This cavity can be supported on fixed sensor areas to improve mechanicalstability.

A further alternative encapsulation technique is presented in U.S. Pat.No. 5,919,364. According to this method, a thin gas-permeablepolysilicon membrane is used as the membrane layer, which can bepenetrated by the reactants during etching of the sacrificial layer.

All methods described above are based on the principle of covering thefunctional elements of the sensor with a further upper sacrificiallayer, which is selectively etched against the functional elements afterapplying a structured membrane layer. The moving parts of the sensor areexposed during this process. This principle has been presented in amodified form, for example in “Electrostatically DrivenVacuum-Encapsulated Polysilicon Resonators: Part I. Design andFabrication”, R. Legtenberg et al., Sensors and Actuators A 45 (1994),57, “The Application of Fine-Grained, Tensile Polysilicon toMechanically Resonant Transducers”, H. Guckel et al., Sensors andActuators A 21-23 (1990), 346, and in the publications cited therein.

Furthermore, German patent documents nos. 100 05 555, 100 06 035, and100 17 422 discuss encapsulation methods in which a thick, stablesilicon layer is used as the cap or cover layer. The object of themethods described in these Offenlegungsschriften was to stabilize thecover layer by using a suitable material (epi-polysilicon in all threecases) having an adequate layer thickness. However, all methods have thedisadvantage that cover layers of an adequate thickness may be reliablyproduced only at great cost and with substantial technical difficulty(for example, topography, mask alignment for photolithography, verticalpath resistances due to doping profiles, lack of homogeneity in depthstructuring of the thick membrane layer (formation of pockets in thecase of trenches), etc.).

The disadvantage of the encapsulation methods which form a thin caplayer is poor cap stability toward stresses during mounting in plasticpackages. For example, an overpressure which may damage the thin caplayer is applied to the material during transfer-molding of the sensors.

SUMMARY OF THE INVENTION

The exemplary embodiment and/or exemplary method of the presentinvention provides a micromechanical component and a method for themanufacture thereof, a micromechanical component structure beinghermetically sealable by a cap structure using only relatively thincover layers. In addition, the component may be packaged in very smallstandard plastic packages, such as PLCC, SOIC, QFN, MLF and CSP.

The exemplary embodiment and/or exemplary method of the presentinvention improves the functionality of micromechanical sensors, sinceparasitic capacitances are reduced, providing greater freedom for theanalyzer circuit. A further advantage of the exemplary embodiment and/orexemplary method of the present invention is that it provides a simplemanner of system-in-package integration, the system function beingtestable on the wafer level.

The exemplary embodiment and/or exemplary method of the presentinvention involves the manufacture of a chip having a cap structure overa chip structure according to an available method, a thin cover layerbeing sufficient—unlike the related art—because the hermeticallyencapsulated chip is mounted according to the exemplary embodimentand/or exemplary method of the present invention on a substrate, e.g.,an analyzer IC, by chip-on-wafer flip-chip assembly with the contactside facing down. In the case of flip-chip assembly, an underfill (usingplastic molding compound/adhesive) is provided between the chip and thesubstrate after bonding and forms the connection between the flip chipsand the substrate in the usual manner. After curing, the underfill alsostabilizes the thin cap structure of the encapsulated chip, in such away that the sensor structure is hermetically protected with a highdegree of reliability against environmental influences and, inparticular, against high insertion pressure during subsequentmold-packaging.

Following chip-on-wafer flip-chip assembly, the chip/substrate systemmay be pretested via metal contacts which are located on the substrateor the chip. During subsequent sawing, the chips are protected by thesubstrate, which may be thick, while the back is hermetically embeddedin the underfill. During further processing, the chip/substrate systemis packaged in plastic as standard procedure.

The high stability despite thin film sensor encapsulation saves moneyduring the sensor process, thus simplifying the sensor technology. Thismakes allows for eliminating a dense support structure of the cap layer,or the density of the supports may be substantially reduced, therebyachieving higher basic capacitances without changing the chip area. Thesystem may be pretested on the wafer level. Low parasitic capacitancesin the electric connection improve functionality.

The thickness of the sensor wafer may be reduced to nearly any thicknessafter encapsulation, for example by precision grinding or chemicalmechanical polishing, since the cap is stable in the CMP step. Thepackage may have a compact arrangement. Compatibility with customers isensured, since standard plastic packages may be used. The slightlyhigher costs of the more complex flip-chip assembly are offset bysavings in sensor production.

According to an exemplary embodiment, the mounting area is a metalplating area, the mounting arrangement including solder bumps forflip-chip assembly.

According to another exemplary embodiment, the substrate is an IC chip.

According to another exemplary embodiment, the chip is a sensor chipand/or actuator chip which has a sensor structure and/or actuatorstructure beneath the encapsulated chip area.

According to another exemplary embodiment, the substrate is mounted on alead frame, the component being surrounded by a plastic package.

According to another exemplary embodiment, the encapsulated chip areahas a cap-type cover for covering a functional area provided on asubstrate, the cap-type cover having at least one perforated cover layer, and the cover layer being sealed by at least one sealing layer.

Although it is applicable to any micromechanical component andstructure, in particular sensors and actuators, the exemplary embodimentand/or exemplary method of the present invention and its underlyingobjective are explained in relation to a micromechanical component,e.g., an acceleration sensor, which may be manufactured on the basis ofsilicon surface micromechanical technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sensor chip in the form of a micromechanical accelerationsensor, which is used in one exemplary embodiment of the presentinvention.

FIG. 2 shows a representation of an IC wafer and a sensor chip to bemounted thereon according to the exemplary embodiment of the presentinvention.

FIG. 3 shows a later phase of the process according to the exemplaryembodiment of the present invention.

FIG. 4 shows the packaging of separated sensor chip/IC chip pairs in aplastic package according to the exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION

In the figures, identical reference numbers designate identical orfunctionally equivalent components.

FIG. 1 shows a sensor chip in the form of a micromechanical accelerationsensor, which is used in a first exemplary embodiment of the presentinvention.

In FIG. 1, reference number 1 identifies a relatively thick siliconsubstrate wafer, which, however, is not drawn to scale in FIG. 1.Reference number 2 is a silicon dioxide sacrificial layer; 3 is afunctional layer made of epi-polysilicon; 4 is a movable structure, forexample electrode fingers; 5 is a perforated cap layer, e.g., made ofepi-polysilicon or LPCVD silicon which is typically 2 μm to 10 μm thickand seals a cavity 11 in which the sensor structure is embedded.Reference number 6 designates a sealing layer made, for example, ofsilicon dioxide, silicon nitride, BPSG, PSG or a similar material whichis typically 2 μm to 8 μm thick. Reference number 7 designates a metalplating layer which has an open metal contact surface 9 for solder bumpsfor the purpose of flip-chip bonding. Reference number 8 designates apassivation layer made, for example, of silicon dioxide or siliconnitride which is typically 200 nm to 1.5 μm thick. Reference number 10designates contact blocks which contact a conductor path level (notillustrated), which, in turn, connects to electrode fingers 4.

In FIG. 1, reference number 18 designates the sensor chip as a whole andreference number 19 the encapsulated chip area which is higher than itsvicinity.

FIG. 2 shows a representation of an IC wafer and sensor chips to bemounted thereon according to the exemplary embodiment of the presentinvention.

In FIG. 2, reference number 15 designates the IC wafer as a whole. ICwafer 15 includes a plurality of IC chips 15 a through 15 e. On IC chips15 a through 15 e, solder bumps 16 are prepared ahead of time in theusual manner for a standard flip-chip process. IC chips 15 a through 15e are usually slightly larger than sensor chips 18 a, 18 b, etc. havingencapsulated areas 19 a, 19 b, etc. Contact pads 17 on IC chips 15 athrough 15 e may therefore be provided outside the area having solderbumps 16, which are used later on for pretesting or wire-bonding duringpackaging.

The representation in FIG. 2 shows the process for mounting sensor chips18 a, 18 b, etc., which may also be pretested separately in the usualmanner, on IC chips 15 a through 15 e, which are still bonded to thewafer and may also be pretested separately to complete flip-chipassembly. According to this flip-chip assembly of sensor chips 18 a, 18b, etc., the sensor chips are mounted in such a way that encapsulatedchip area 19 a, 19 b, etc. is surrounded by solder bumps 16 and ispositioned at a distance from the surface of IC chips 15 a through 15 e.In this regard, solder bumps 16 may be provided on sensor chips 18 a, 18b, etc. instead of on IC chips 15 a through 15 e.

FIG. 3 shows a later phase of the process according to the exemplaryembodiment/method of the present invention.

According to FIG. 3, all sensor chips 18 a through 18 e are nowflip-chip-bonded to corresponding IC chips 15 a through 15 e. Followingflip-chip bonding, an underfill 20 made of a plastic molding compound ora plastic adhesive is placed in the gap between a particular sensor chip18 a through 18 e and associated IC chips 15 a through 15 e. This isusually carried out via a dispensing step in which capillary forces drawthe underfill between sensor chips 18 a through 18 e and IC chips 15 athrough 15 e. Underfill 20 is then cured, and it increases the stabilityof the flip-chip bond. In addition, underfill 20 stabilizes the thin capmembrane during later assembly in the plastic package. After underfill20 has been cured, the system may be pretested on the wafer level, sinceelectric contacts 17 are freely accessible.

The main advantage of underfill 20 is that it may be applied largelywithout overpressure and therefore places no stress on theencapsulation. After curing, the underfill stabilizes the encapsulationin that, during injection molding, it is supported on the stationarysensor areas or the surrounding area against the mold pressure. Inaddition to traditional underfill materials, any materials may be usedwhich are initially applicable without pressure and then curable in asubsequent crosslinking step (heat-curing, cross-linking by moisture,etc.). The thermal expansion coefficient of underfill 20 isadvantageously matched to that of the silicon of the sensor chip or ICchip.

In another method step, the sensor chip/IC chip pairs may finally beseparated by a sawing process.

FIG. 4 shows the packaging of the separated sensor chip/IC chip pairs ina plastic package according to the exemplary embodiment of the presentinvention.

In FIG. 4, reference number 22 designates a lead frame on which the ICchip/Sensor chip pair is mounted, for example by soldering. Referencenumber 25 identifies bonds from the inner area of lead frame 22 to theouter area. Reference number 30 designates the plastic package which ismolded around the assembly structured in this manner. Very highhydrostatic pressures of up to 100 bar occur during molding. During thisprocess, underfill 20 protects the thin sensor encapsulation and absorbsthe pressure. The sensor structure is protected on top by substratewafer 1. Substrate deflection is minimal and determines the maximumexpansion of the thin sensor encapsulation. In addition, solder bumps 16act as rigid spacers and reduce the deflection of the sensor chip andthus also that of the thin sensor encapsulation. Solder bumps 16 areadvantageously positioned in such a way that a predefined sensor chipstructure ensures optimum stability. In this assembly, the sensorstructure is hermetically protected against environmental influences andhigh pressures. In addition, the thermal expansion coefficients of theunderfill and plastic package 30 are matched to each other to the extentpossible. As a result, no critical strains occur later on during changesin temperature.

Although the present invention was described above on the basis of anexemplary embodiment(s), it is not limited thereto, but is modifiable ina number of different ways.

In particular, any micromechanical base materials may be used, and notonly the silicon substrate described by way of example.

The exemplary method according to the present invention may be used, inparticular, for any sensor and actuator elements manufactured by surfacemicromechanical or bulk micromechanical methods. For example, sensor oractuator structures having an integrated analyzer circuit may be mountedon a chip and the latter may be packaged with a further ASIC.

Although the mounting area in the above example is a metal plated areaand the mounting arrangement includes solder bumps for flip-chipassembly, other assembly types, for example anisotropic or isotropicadhesion or thermocompression welding, etc. may also be used.

The list of reference numbers is as follows:

1 Substrate wafer

2 Sacrificial layer

3 Polysilicon functional layer

4 Electrode fingers

5 Cap layer

6 Sealing layer

7 Contact pad

8 Passivation layer

9 Metal contact surface

10 Contact spot

11 Cavity

15; 15 a-e Substrate, IC wafer

16 Solder bumps

17 Contact pads

18; 18 a-e Sensor chips

19; 19 a-e Encapsulated area

20 Underfill

22 Lead frame

25 Bonding wire

30 Plastic package

1-17. (canceled)
 18. A micromechanical component comprising: a chipmounted on a substrate, and having an encapsulated chip area which ishigher than its vicinity, a mounting area being provided in a vicinityof the encapsulated chip area; wherein the chip is mounted on thesubstrate using a mounting arrangement which is connected to themounting area, so that the encapsulated chip area faces the substrateand is positioned at a distance therefrom, the encapsulated chip areabeing surrounded by an underfill beneath the chip.
 19. Themicromechanical component of claim 18, wherein the mounting areaincludes a metal-plated area, and the mounting arrangement includessolder bumps for a flip-chip assembly.
 20. The micromechanical componentof claim 18, wherein the mounting area includes an adhesive area, andthe mounting arrangement includes an adhesive arrangement.
 21. Themicromechanical component of claim 18, wherein the mounting areaincludes a welding area, and the mounting arrangement includes a weldingzone.
 22. The micromechanical component of claim 18, wherein thesubstrate includes an integrated circuit chip.
 23. The micromechanicalcomponent of claim 18, wherein the chip includes at least one of asensor chip, an actuator chip which has a sensor structure, and anactuator structure beneath the encapsulated chip area.
 24. Themicromechanical component of claim 18, wherein the substrate is mountedon a lead frame, and the component is surrounded by a plastic package.25. The micromechanical component of claim 18, wherein the encapsulatedchip area includes a cap-type cover for covering a functional areaprovided on a substrate, the cap-type cover having at least oneperforated cover layer which is sealed by at least one sealing layer.26. A method for making a micromechanical component, the methodcomprising: providing a chip which includes an encapsulated chip areawhich is higher than its vicinity, and a mounting area in a vicinity ofthe encapsulated chip area; mounting the chip on a substrate via amounting arrangement, which is connected to the mounting area, so thatthe encapsulated chip area faces the substrate and is positioned at adistance therefrom; and underfilling the chip so that the encapsulatedchip area is surrounded by an underfill beneath the chip.
 27. The methodof claim 26, wherein the mounting area includes a metal-plated area, andthe mounting arrangement includes solder bumps for a flip-chip assembly.28. The method of claim 26, wherein the mounting area includes anadhesive area, and the mounting arrangement includes an adhesivearrangement.
 29. The method of claim 26, wherein the mounting areaincludes a welding area, and the mounting arrangement includes a weldingzone.
 30. The method of claim 26, wherein the substrate includes anintegrated circuit chip.
 31. The method of claim 30, wherein a pluralityof chips are mounted on a plurality of wafer-bonded IC chips, and thecomponents are subsequently separated.
 32. The method of claim 26,wherein the chip includes at least one of a sensor chip, an actuatorchip which has a sensor structure, and an actuator structure beneath theencapsulated chip area.
 33. The method of claim 26, wherein thesubstrate is mounted on a lead frame, and the component is surrounded bya plastic package.
 34. The method of claim 26, wherein the encapsulatedchip area includes a cap-type cover for covering a functional areaprovided on the substrate, the cap-type cover including at least oneperforated cover layer which is sealed by at least one sealing layer.